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Re: [Discuss-gnuradio] Queries regarding FPGA

From: Eric Blossom
Subject: Re: [Discuss-gnuradio] Queries regarding FPGA
Date: Mon, 27 Feb 2006 09:04:54 -0800
User-agent: Mutt/1.5.9i

On Mon, Feb 27, 2006 at 08:28:56AM -0800, Thomas Schmid wrote:
> Isn't one of the problems that Linux is not a real time os, i.e.,
> exact time scheduling is impossible? As far as I know, linux timers
> have an accuracy of 1ms (i.e. 1 jiffie in the kernel). If you want to
> have shorter delays, you use the udelay function and do a busy wait.
> Or is there something that I am missing?

This is basically true.  However, it's possible to implement finer
timing granularity with the assistance of external hardware, e.g., the
USRP.  One could imagine sending a packet of data to the USRP with a
header on it that said, "Do not transmit until time = X", where time
is measured relative to a high speed counter on the board.

Also, part of what is being discussed is getting rid of excess
latency.  This will shorten the typical round-trip loop time.  It
doesn't have to be "hard-real time", just good enough.  It's also
possible to design MAC's that don't require extremely tight control
loops.  802.11 a/b/g RTS/CTS isn't one of them.  Designing better MACs
is a very active research topic.


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